2airishuman
Contributor
Early in the tech era, there were extensive disagreements on the best gas to use for the shallow stops with advocates for pure O2 and 75%. What is the current practice? Does it still vary by agency?
Current best practice for deco gas: 75% vs 100%
- Thread starter 2airishuman
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If you run the numbers, there is little gained between 50 and 75 being used. My classes just a few years back (NAUI. but I believe it to be my instructor) basically listed 50 and 100 as the choices. If you aren't "spot on" with holding the stop, or conditions are adding risk, my learning said 50%.....
Now, with adding He, it was 100 or nothing....
Curious as to what others will chime in with...
Now, with adding He, it was 100 or nothing....
Curious as to what others will chime in with...
lv2dive
Formerly known as KatePNAtl
I’ve taken tech classes with GUE, TDI, and IANTD... used 100% in all of them.
TrimixToo
Contributor
Assuming you have not run up to 100% CNS, at 20' and shallower the rate of offgassing should be proportional to the differential partial pressure between inspired gas and dissolved gas in the blood at the lungs. It's hard to do better than 100% differential, so assuming you are not risking oxtox, 100% is always better that shallow IMO. Deeper stops might benefit from lower O2 percentages but having played with this a bit with planning software, the benefits don't seem meaningful unless you want to add a gas to what you carry.
What agency or organization ever advocated 75%? (or is this a misprint of 80%?)
I’ve taken tech classes with GUE, UTD, and NAUI... used 100% in all of them.
I know a few CCR divers using a deep bailout gas and 70% as their single deco gas, because depending on the profile 40ft is often where the significant stops start (for them). I don't think they are bringing enough deep BO to actually get to 40ft... But that turns into a more complicated discussion that I don't think you were asking.
I’ve taken tech classes with GUE, UTD, and NAUI... used 100% in all of them.
I know a few CCR divers using a deep bailout gas and 70% as their single deco gas, because depending on the profile 40ft is often where the significant stops start (for them). I don't think they are bringing enough deep BO to actually get to 40ft... But that turns into a more complicated discussion that I don't think you were asking.
PfcAJ
Contributor
100%.
It has no inert gas.
That’s what’s good about it.
It has no inert gas.
That’s what’s good about it.
This is the first I have ever heard of anyone using 75%.
For final deco gas, I use 100% at every opportunity, which in my case is pretty much 100% of the time. Using less O2 deco gases (like 80%) for the final stop makes some sense when your transfill situation makes it difficult to get enough O2 to make it work, but I don't see any reason to aim for it intentionally.
For final deco gas, I use 100% at every opportunity, which in my case is pretty much 100% of the time. Using less O2 deco gases (like 80%) for the final stop makes some sense when your transfill situation makes it difficult to get enough O2 to make it work, but I don't see any reason to aim for it intentionally.
50% is good from a longish deepish OC dive, because it's better than back gas and you can get on it at 70 ft. 80% (75% would be the same, but I've never heard of anyone using it) is good if surface conditions in open water are terrible, and you can't get to 20ft and hold it to use 100%. My dives on the USS Monitor used 50% and 100% to limit total deco time, and we only dived when conditions were pretty good.
ajduplessis
Contributor
I dont like 50%, it does most things poorly.
I'm a big fan of 50% or 60% for a few reasons ( I do not do trimix and most dives using deco gas are 30-50MT)
- I do not have a booster so I can get 200 bar fills with 50-60% ( more gas always better)
- On long deep dives I can start off gasing deeper and I use more of my deco gas.
- I can top up the leftover and have a high nitrox for a shallow dive so no wasted gas.
- if needed I can always take a 100% along as well but I'm happier with less tanks
- I do not have a booster so I can get 200 bar fills with 50-60% ( more gas always better)
- On long deep dives I can start off gasing deeper and I use more of my deco gas.
- I can top up the leftover and have a high nitrox for a shallow dive so no wasted gas.
- if needed I can always take a 100% along as well but I'm happier with less tanks
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